Poly-p-xylylene polymer ("PPX") coatings are useful in many applications for providing electrical insulation and moisture barriers. Because they are applied in a vapor deposition polymerization process, the coatings can be thin and extremely uniform in coverage. The ability of the polymer molecules to form void-free films on the substrates in the polymerization chamber is the key factor in the excellent encapsulation qualities of the polymer.
Unfortunately, the PPX coatings are unable to form chemical bonds with the surfaces of the substrates. Wet chemical coupling agents are commonly used in the industry to provide a-measure of adhesion. However, their use has raised environmental concerns.
The preparation of polymer surfaces before application of PPX by gas plasma cleaning or activation is well known. Many PPX coating facilities routinely employ gas plasma to clean the substrate surfaces prior to PPX coating. A few have demonstrated the ability to create a gas plasma within the PPX polymerization chamber itself, thereby forming a high concentration of radicals on the surfaces to be coated. Since the vacuum chamber need not be opened to air between the plasma and the PPX processes, the lifetime of these surface radicals is relatively long, and the PPX molecules have an opportunity to form chemical bonds to the substrate surface. This type of surface preparation, although useful for substrates made of polymeric materials, is not very effective when applied to direct PPX coating of metals or other inorganic surfaces such as ceramics or semiconductors.
The use of glow discharge polymer ("GDP" or plasma polymer) as insulative coatings and as primary layers for the subsequent application of PPX is well documented in the literature. It has been demonstrated that use of GDP, specifically of methane, as "primer" coatings on metals to improve the adhesion of the PPX to the substrates. The adhesion improvement and concomitant increased moisture resistance is due to the creation of a highly crosslinked, hydrophobic film via GDP. Several monomer gases were proven to be effective, notably methane, ethane, propane, ethylene, and propylene.
However, few reports, if any, hitherto have addressed the issue regarding the practical use of GDP as a primary layer prior to PPX coating. As a matter of fact, most of the previously described processes exhibit distinct drawbacks which would render them unusable in an industrial process.